1. Field of the Invention
This invention relates to floating vessels requiring the input of large volumes of cold seawater to support onboard processes—e.g., Ocean Thermal Energy Conversion (OTEC) facilities, floating natural gas liquefaction plantships, and the like. More particularly, it relates to the Cold Water Pipe (CWP) used to bring cold water up from depth in such facilities.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The OTEC (Ocean Thermal Energy Conversion) process uses the temperature difference between warm surface seawater and cold, deep seawater (˜1000 m below the surface) to power a heat engine and produce useful work, usually in the form of electricity. The most commonly used heat cycle for OTEC is the Rankine cycle using a low-pressure turbine. Systems may be either closed-cycle or open-cycle. Closed-cycle engines use working fluids that are typically thought of as refrigerants such as ammonia, R-134a or organic fluids. Open-cycle engines use vapor from the seawater itself as the working fluid. OTEC facilities may generate electricity and desalinated water as well as other energy carriers such as hydrogen (H2) and ammonia (NH3).
The OTEC process requires one or more large diameter Cold Water Pipes (CWPs) connected to a floating power plant that extend to ocean depths of approximately 500 to 1000 meters. The CWP is a conduit for the delivery of cold water to the surface, and the CWPs are sized on the order of 4 meters in diameter for each 10 MW of power generation capacity.
The large diameter CWP may be rigidly constructed of steel, thermoplastics, fiberglass reinforced plastic (FRP) or other suitable materials. The CWP is subjected to the first and second order motions of the surface floater, ocean currents and submerged wave forces, as well as hydraulic pressure differentials.
The CWP must gimbal freely while connected to the surface vessel at quasi-static and dynamic angles of approximately 10 to 20 degrees with respect to the vertical axis. During these conditions the CWP must continually deliver cold water to the surface power plant with minimal flow restrictions in the pipe inner diameter. The gimbal must therefore provide an effective seal and support the CWP self-weight, inertial, and drag forces while gimballing with respect to the surface vessel. The gimbal seal must withstand differential pressures of approximately 1.7 bar, which are normally positive on the outside of the CWP, but can also be positive on the inside of the CWP under some conditions.
Gimballing connections have been provided for Steel Catenary Risers (SCRs) to surface floaters which are capable of sealingly transporting fluids under pressure such as oil, gas and water. These pipeline connections are usually on the order of 8 to 24 inches in diameter. However, the gimballing connection of the OTEC CWP poses unique problems due to its order-of-magnitude larger diameter. This can be solved by keeping the gimballing structural connection near the center of the CWP, but the seal must withstand a relatively high pressure at the CWP outer diameter, while accommodating large radial excursions on the order of +/−1 to 2 meters.
U.S. Pat. No. 7,373,986 discloses a riser connector that connects the upper portion of a riser having a latching enlargement to a receptacle on a vessel, so the riser upper end can connect to a fluid coupling leading to a conduit on the vessel. The enlargement has a diameter larger than that of the riser and encircles the riser. A double-click mechanism, with parts on the enlargement and on the receptacle, turns a load ring on the enlargement to latch the enlargement in place when it is lifted to an upper position and lowered to a latched position, and releases it by again lifting and lowering. A cable coupling is temporarily mounted on the top of the riser to enable the riser to be lifted by a cable, the cable coupling being later removed to connect the riser upper end to the fluid coupling on the vessel.